For more than half a century, the cathode ray tube (CRT) has been the principal electronic device for displaying visual information. The widespread usage of the CRT may be ascribed to the remarkable quality of its display characteristics; namely, color, brightness, contrast and resolution. One major feature of the CRT permitting these qualities to be realized is the use of a luminescent phosphor coating on a transparent faceplate.
Conventional CRT's, however, have the disadvantage that they require significant physical depth, i.e., space behind the actual display surface, making them bulky and cumbersome. They are fragile and, due in part to their large vacuum volume, can be dangerous if broken. Furthermore, these devices consume significant amounts of power.
The advent of portable computers has created intense demand for displays which are light-weight, compact and power efficient. Since the space available for the display function of these devices precludes the use of a conventional CRT, there has been significant interest in efforts to provide satisfactory so-called "flat panel displays" having comparable or even superior display characteristics, e.g., brightness, resolution, versatility in display, power consumption, etc.
Currently, liquid crystal displays (LCD's) are used almost universally for laptop and notebook computers. In comparison to a CRT, these displays provide poor contrast, only a limited range of viewing angles, and, in color versions, they consume power at rates which are incompatible with extended battery operation. In addition, color screens tend to be far more costly than CRT's of equal screen size.
As a result of the drawbacks of liquid crystal display technology, thin film field emission display technology has been receiving increasing attention by industry. Flat panel displays utilizing such technology employ a matrix-addressable array of pointed, cold field emission microtips in combination with an anode comprising a phosphorluminescent screen. The phenomenon of field emission was discovered in the 1950's, and extensive research by many individuals, such as Charles A. Spindt of SRI International, has improved the technology to the extent that its use in the manufacture of inexpensive, low-power, high-resolution, high-contrast, full-color flat displays has been realized.
Advances in field emission display technology are disclosed in U.S. Pat. No. 3,755,704, "Field Emission Cathode Structures and Devices Utilizing Such Structures," issued Aug. 28, 1973, to C. A. Spindt et al.; U.S. Pat. No. 4,940,916, "Electron Source with Micropoint Emissive Cathodes and Display Means by Cathodoluminescence Excited by Field Emission Using Said Source," issued Jul. 10, 1990 to Michel Borel et al.; U.S. Pat. No. 5,194,780, "Electron Source with Microtip Emissive Cathodes," issued Mar. 16, 1993 to Robert Meyer; and U.S. Pat. No. 5,225,820, "Microtip Trichromatic Fluorescent Screen," issued Jul. 6, 1993, to Jean-Frederic Clerc. These patents are incorporated by reference into the present application.
The conventional process for forming the microtips in the emitter plate of the flat panel display is taught by the Spindt et al. ('704) patent. This process involves forming a sacrificial layer, called a lift-off layer, on the surface of the gate using low angle evaporation techniques well known in the industry. The lift-off layer is illustratively nickel. The microtips are formed by evaporation, at a normal angle, of the tip metal into the holes formed in the gate metal and underlying insulator material. The tip metal is illustratively molybdenum. The lift-off layer is then dissolved by an electrochemical process which then exposes the gate metal and the microtips.
There are many disadvantages associated with forming the lift-off layer using low angle evaporation. First, the long throw angle dictates the need for a very large evaporation chamber. The use of large evaporation chambers slows the manufacturing time of flat panel displays because opening the large chamber doors to load the emitter plates exposes the whole chamber to atmospheric pressure. The increase in manufacturing time increases the cost of the flat panel display and therefore reduces the ability to compete in price in the marketplace.
In view of the above, it is clear that there exists a need for an improved microtip deposition process. More specifically, what is needed is a microtip deposition process which overcomes the disadvantages of using the low angle evaporation process.